Metallic coating, pearl color coating, and other coating for use in car coating may appear to vary in color depending on an observer's angle due to a radiant material of the interior. For this reason, a multi-angle colorimeter that performs illumination or light receiving at multiple angles is used for coating evaluation (evaluation of coating color).
In other words, in metallic coating and pearl color coating for use in car coating, coating films contain aluminum flakes and mica flakes referred to as radiant materials, which produce a so-called metallic effect and pearl effect. This is because the radiant material contributes to reflection characteristics differently depending on the directions of illumination and observation. A multi-angle colorimeter having a multi-angle geometry (optical arrangement) for illuminating a sample surface of an object to be measured from multiple directions and receiving the light from one direction (multidirectional illumination and unidirectional light receiving) or illuminating a sample surface of an object to be measured from one direction and receiving the lights from multiple directions (unidirectional illumination and multidirectional light receiving) is used in the evaluation (colorimetry) of metallic coating and pearl color coating.
Unfortunately, in a case where an object to be measured is a sample having a curvature, such as a car bumper, an attitude error is highly likely to occur, where a sample normal does not coincide with a reference axis of a colorimeter in measurement. Among others, an angular orientation close to the specularly reflected light has large angle dependence in the reflection characteristics, and thus, an effect of this error is not negligible.
In order to reduce the attitude error, for example, the method disclosed in Japanese Patent Application Laid-Open No. 2002-5830 proposes the technology in which an optical base unit containing a measurement optical system is held by an elastic body such as a spring against a housing, and the illumination and light receiving geometories are kept constant irrespective of the contact angle between the sample and the housing, to thereby reduce an attitude error.
An example of another well-known technology is the technology in which multiple contact pins are arranged on a sample contact surface such that a measurement is triggered if those pins are pressed evenly, to thereby suppress an attitude error of a colorimeter.
Meanwhile, the method disclosed in Japanese Patent Application Laid-Open No. 2007-315761 proposes the technique in which in a glossimeter that measures a gloss of a sample from the specularly reflected light, an optical system for correction is additionally arranged to be axisymmetric with the original illumination system and light receiving system about a sample normal for suppressing an attitude error of the glossimeter, to thereby average measured values of the both systems.